1. Field of the Invention
This invention relates generally to thermal dispersion sensors and more particularly to such sensors employing thermally sensitive detection elements mounted externally to the fluid container or conduit to sense liquid level therein or mass flow rate therethrough. Alternatively, an insertion device can also be employed wherein the detectors are inside the vessel but, because of various sealing mechanisms, are still topologically outside the container and isolated from the media.
2. Discussion of the Related Art
In many industrial and commercial fields there is a requirement for compact and versatile flow rate detectors which positively determine that a particular mass of fluid is flowing, has stopped flowing, is flowing above or below a predetermined threshold mass velocity level or the actual mass flow rate at which it is flowing. Alternatively, such a device may be used to determine when the level of rising or falling liquid in a container has reached a predetermined height. The conduits and containers involved may be oriented vertically, horizontally, or they may be inclined, and can range in size from fractions of an inch to as much as several feet in diameter. Returning to the mass flow rate sensing application, this requirement is particularly strong in manufacturing situations where it is critical that the amount and velocity of flowing gas must be known. This is even more critical where those gases are toxic, which often occurs in the manufacture of electronic chips.
Fabrication methods incidental to the mechanical operating principles of current devices used in the electronic chip industry often result in dead-end cavities, labyrinthian passageways, irregular and rough surfaces from welding, close diametral clearances between moving parts. The impossibility of purging and other effects from the labyrinthian passages are also detrimental to delivering clean gas. In many instances, the absence of polished surfaces, the presence of close fitting parts and dead-end cavities can all but prevent the delivery of pure products or the purging of the system when gaseous products are changed. Moreover, current devices used in chip manufacturing typically employ moving parts. Inevitably, particle matter generated by the moving parts further contaminate the gaseous media being employed. Ultra cleanliness and purity are absolutely necessary if high quality electronic chips are to be manufactured. In many instances, smooth finishes and the ability to effectively clean the conduit can be all but impossible to achieve with some devices currently on the market. Failure to note that flow has ceased or has been reduced below or increased above certain predetermined flow velocity thresholds in a conduit may be very costly and in some instances could be catastrophic. The same can be true of liquid levels. As processes increase in speed and output, and precision becomes ever more of a requirement, often resulting from advances in technology, such failures tend to be ever more costly and dangerous.
Devices have long been available for detecting and, in some cases, measuring the rate of flow of fluids or liquid level. A common type of flow detector utilizes the force exerted by the moving fluid against a paddle or movable wall immersed in the fluid or the fluid flow to indicate or determine the rate of fluid motion. Regardless of the form chosen for the immersed object, for example, propeller, vane, piston, deflection arm, drogue or the like, all of these devices are subject to a number of potentially serious shortcomings for certain uses, especially for mass flow rate measurements as required in chip manufacturing and most other applications. All of the above require compensation for pressure and temperature and the effects these variables have on density of gaseous media. Moveable parts tend to deteriorate after continued immersion for extended periods of time and can become corroded or frozen in place after even brief contact with many fluids. This is especially true with gases or liquids which may be toxic, corrosive, or both. Seals and packing, always at least minor problems, become monumental tasks when moving parts are involved. Mechanical deformation and fatigue induced breakdowns also plague this class of indicators. When these mechanical devices are used, they are, by and large, wholly unsuitable for chip manufacturing purposes as well as many other exacting purposes. They are also generally unsuitable for the detection of flow stoppage, reduction in flow velocity below a predetermined level, or changes in fluid level in customary commercial and industrial applications. This subject will be further discussed below.
Because of the disadvantages of requiring the force exerted by the moving fluid against some object in order to provide detectable motion or level changes, thermal dispersion mass flow meters have become a common choice for flow metering devices in the commercial and industrial metering and level sensing markets. A typical flow sensor element for use in such meters is the resistance temperature detector (RTD), the resistance of which is related to the temperature of the element itself. Although RTDs are the preferred device, many other types of small heatable temperature detector/heaters could also be considered. Thermocouples, thermistors, temperature sensitive diodes and other transistors or solid state devices could be used. Also RTDs come in many forms such as chips, wire wound elements and grids. A typical flow rate or level sensor employs at least two RTD elements. One of them is referred to as a reference element and is normally unheated. The active RTD element is heated and the temperature reduction effect of mass flow or wetting on the heated element provides a measure of the mass flow velocity or a phase change from dry to wet of the substance in the conduit or vessel being monitored. The density of a gaseous fluid flowing across the active RTD is also a directly proportional factor in the amount of heat dissipated from the RTD. As discussed above, RTD sensors can also be employed for liquid level detection and interface detection of gas to liquid, and non-miscible liquids such as oil/water, clear water and sludge or slurries, to name a few.
There are many configurations of dispersion mass flow sensors, and more particularly, of heated RTD type sensors. An early such flow detector is found in U.S. Pat. No. 3,366,942. This patent discloses a reference sensor, a heated or active sensor, and a separate heating element located closely adjacent the heated sensor element. The basic principle of operation of dispersion flow meters is well known and is discussed in this patent. A different configuration of a three-element thermal dispersion sensor is shown in U.S. Pat. No. 4,899,584. There any many other examples of detectors employing differential temperature sensors, some having three elements as described in the patents mentioned above, and some having two elements, where the active sensor has the heater integral therewith and is self heated. Even a single element differential temperature sensor may be employed. The single element sensor works on a time sharing basis where it acts as a reference sensor part of the time and is then heated to act as the active sensor, switching alternately in relatively rapid succession. Another example of a differential temperature sensor is shown in U.S. Pat. No. 5,780,737.
The devices shown in the patents mentioned above have no moving parts and have proven satisfactory, at least in many circumstances where it is desirable to determine that fluid flow has stopped. They are also very sensitive to low levels of mass flow of fluid. It is important to note that in the examples above and in many other related examples, the RTD type sensors are mounted in a thermal well and are immersed in the fluid flowing in a conduit, or are positioned to be wetted by liquid at predetermined levels.
For the manufacture of electronic chips, where toxic gases are employed, immersion sensors of any type are generally not appropriate because of the intrusion of the expensive thermal wells into the relatively small conduit containing the flowing stream. Such devices typically are too large for the conduits involved in electronic chip manufacture and likely cannot be properly purged of possible residual gases from previous uses.
For example, in electronic chip manufacturing, noxious and often toxic gasses are used in vapor deposition. In order to control the flow of those vapors and to ensure that excess vapors do not overload the system's capacity to properly contain them, an excess flow sensor and switch can be used. Examples of prior art devices which can be employed for such purposes are flow rate magnet/reed switches. Magnetic switches of this type are sold by Nupro Company under the designation "FV4 Series Vertical Flow Sensor," and the series "AP74 Vertical Flow Switch" is sold by Advanced Pressure Technology, specifically for use in the manufacture of electronic chips. An actual switch is required, which is external to the conduit through which the gases flow, and a moving magnet is positioned within the fluid conduit. Thus these devices are partially direct contact and partially remote sensor devices. These mechanical switches are not well suited for mass flow rate sensing because they are sensitive to volume flow rate and mass flow rate errors are introduced because of density variables. Principally, pressure is the primary cause of such density variables. For these types of switches, several different models would be required to satisfy the various trip points that might be specified by any user. The trip point of choice is fixed and set in the factory and is not accurate as explained in their specifications.
Temperature can also affect density and is a contributing error factor in some cases. Trade literature for such magnetic switch products show that the trip point flow rate is a function of pressure. One model will trip at 15 SLPM (standard liters per minute) when the system is pressurized to 100 psig. If this model were placed in a 20 psig system, it would trip at 7 SLPM. That is more than a 100% difference from a trip point of 15 SLPM. Thus the requirement of many different models for different flow rates and density uses. Ideally, the customer would prefer to have a single switch with a trip point of, for example, 10 SLPM, which would trip at that value over any pressure range between 0 and 100 psig. Also, these magnetic switches have a wide hysteresis where the trip point has a very different value than the reset point of a particular switch.
Not only is it all but impossible to achieve appropriate cleaning and smooth finishes, but welding, purging and other effects from the labyrinthian passages of the magnetic sensors discussed above can be detrimental to the delivery of clean gas. Additionally, the moving magnet and its enclosure in the flowing media may also generate foreign particles which could contaminate the electronic chips being manufactured.
There are some external or conduit surface mounted temperature detectors previously available. An example is the series AP7300 Flow Switch by Advanced Pressure Technology and the Rheotherm Flow Instruments of Intek, Inc. These are indeed external surface mounted devices but there is no indication of the existence of a local, small, specially prepared surface to increase the sensitivity of the sensing element, at low power levels, to the rate of flow of the fluid within the conduit to which they are attached. Additionally, a separate heater is employed and miniaturization by use of a chip type RTD is not shown. Another example is the use of wire RTDs wrapped around small and medium size tubes as the sensor element. Such a sensor would be relatively large in area and require high power for heating. If the conduit is thinned on its periphery to bring the wire wound RTD closer to the flowing medium, it would compromise the mechanical strength of the tube. Power requirements can be relatively high and sensitivity may be insufficient for detecting small flow rate changes, especially when the mass flow rate is low.
It is readily understood that whenever the substance being measured is in direct contact with the measuring instrument, the measuring instrument will have some effect upon the substance being measured. Thus, there is a need for a sensitive detector for liquid level or fluid flow to accurately and sensitively measure fluid flow or level without directly affecting or being in direct contact with the fluid being measured. A general purpose industrial thermal flow switch employing thermal wells, such as the Model FLT-93S manufactured and sold by Fluid Components Intl, is generally inappropriate for the particular application (electronic chip manufacturing) to which one example of the present invention is most specifically directed. Further, such devices are inappropriate for most other purposes of the invention described below because they have relatively large heaters and consequently require relatively high operating power. Additionally, those devices do not employ laminar boundary layer flow sensing.